Loops Lecture 12, Tue Feb 21 2006

1
Loops Lecture 12, Tue Feb 21 2006
based on slides by Kurt Eiselt
http//www.cs.ubc.ca/tmm/courses/cpsc111-06-spr
2
News

• Welcome back!
• resume lectures, labs, tutorials, office hours
• Midterm and Assignment 1 returned
• pick up after class if you don't have yet
• midterm solutions posted on WebCT
• Assignment 2 posted soon
• probably later today

3
Reading

• This week Chapter 7 all (7.1-7.4)

4
Recap Comparing Strings

• Relational operator is wrong way to compare
• String name1 "Bubba"
• String name2 "Bubba"
• System.out.println(name1 name2) // prints
  false
• equals method is right way to compare Strings
• String name1 "Bubba"
• String name2 "Bubba"
• System.out.println(name1.equals(name2)) //
  prints true
• why? diagrams will help

5
Recap Comparing Strings

• name1 name2 two different references, false

"Bubba"
name1
name2
"Bubba"

• name1.equals(name2) contents same, true

6
Recap Short-Circuting Evaluation

• Java evaluates complex expressions left to right
• short-circuiting Java stops evaluating once
  value is clearly true or false
• aka lazy evaluation
• if ((b gt a) (c 10))
• System.out.println("when blta
  short-circuit")
• if ((b gt a) (c 10))
• System.out.println("when bgta short-circuit")
• Corollary avoid statements with side effects
• if ((b gt a) (c))
• System.out.println("Danger Will Robinson!")

7
Recap Conditional Syntax

• if ( boolean expression ) statement
• else if ( boolean expression ) statement
• optional zero, one, or many
• else statement
• optional
• if, else are reserved words
• parentheses mandatory
• statement can be
• single line
• block of several lines enclosed in

8
Recap Comparing Floats/Doubles

• Relational operator for equality not safe for
  floating point comparison
• if (.3 1.0/10.0 1.0/10.0 1.0/10.0))
• System.out.println("Beware roundoff error")
• Check if difference close to 0 instead
• if (Math.abs(f1 - f2) lt TOLERANCE)
• System.out.println (Essentially equal.)

9
Recap Comparing Characters

• Safe to compare character types with relational
  operators
• char c 'a'
• char d 'b'
• if (c d)
• System.out.println("they match")

10
Recap Switch Syntax

• switch ( expression )
• case value
• statements
• break
• case value
• statements
• break
• default
• statements
• switch, case, break are reserved words
• expression and value must be int or char
• value cannot be variable
• break important, or else control flow continues
  to next set
• statements can be one line or several lines
• default executed if no values match expression

11
Objectives

• Practice with conditionals
• Understand basic loops

12
public class NestTest3 public static void
main (String args) respondToName("Floc
inaucinihilipiliphication")
respondToName("Supercalifragilisticexpialidocious"
) respondToName("Ambrose")
respondToName("Kermit")
respondToName("Miss Piggy!!!")
respondToName("Spot") respondToName("me")
public static void respondToName(Strin
g name) System.out.println("You're
named " name) if (name.length() gt 20)
System.out.println("Gosh, long
name") System.out.println("Keeping
typists busy...") else if
(name.length() gt 30)
System.out.println("Over the top")
else if (name.length() lt 10) if
(name.charAt(0) 'A')
System.out.println("You're first")
else if (name "Kermit")
System.out.println("You're a frog")
System.out.println("I love animals")
else if (name.equals("Spot"))
System.out.println("You're spotted")
else if (name.length() lt 3)
System.out.println("Concise!")
13
Repetition, Iteration, Loops

• Computers good at performing same task many times
• Loops allow repetitive operations in programs
• aka iteration statements, repetition statements
• Loops handy in real life too

14
Climbing Stairs

• Am I at the top of the stairs?

15
Climbing Stairs

• Am I at the top of the stairs?
• No.
• Climb up one step.

16
Climbing Stairs

• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?

17
Climbing Stairs

• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.

18
Climbing Stairs

• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?

19
Climbing Stairs

• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• ...and so on...

20
Washing Hair

• Lather

21
Washing Hair

• Lather
• Rinse

22
Washing Hair

• Lather
• Rinse
• Repeat

23
Washing Hair

• Lather
• Rinse
• Repeat
• When do you stop??

24
While Statement

• while (boolean expression)
• body
• Simplest form of loop in Java
• Body of loop can be
• single statement
• whole block of many statements in curly braces
• Control flow
• body executed if expression is true
• then boolean expression evaluated again
• if expression still true, body executed again
• repetition continues until expression false
• then processing continues with next statement
  after loop

25
If Versus While Statements
how if statement works
boolean expression
true
false
statement
26
If Versus While Statements
how while statement works
how if statement works
boolean expression
boolean expression
true
true
false
false
statement
statement
27
If Versus While Statements
how while statement works
how if statement works
boolean expression
boolean expression
true
true
false
false
statement
statement

• How can loop boolean change from false to true?

28
If Versus While Statements
how while statement works
how if statement works
boolean expression
boolean expression
true
true
false
false
statement
statement

• These diagrams called flowcharts

29
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• while statement

30
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• boolean expression

31
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• while statement body

32
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• statement after while
• control flow resumes here when boolean is false

33
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• trace what happens when execute

34
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
35
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
1
36
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
1
Is counter lt limit? yes
37
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
1
Is counter lt limit? yes
"The square of 1 is 1" printed on monitor
38
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
2
39
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
2
Is counter lt limit? yes
40
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
2
Is counter lt limit? yes
"The square of 2 is 4" printed on monitor
41
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
3
42
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
3
Is counter lt limit? yes
43
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
3
Is counter lt limit? yes
"The square of 3 is 9" printed on monitor
44
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
4
45
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
4
Is counter lt limit? NO!
46
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")

limit
3
counter
4
Is counter lt limit? NO!
End of demonstration" printed on monitor
47
Climbing Stairs Again

• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• Am I at the top of the stairs?
• No.
• Climb up one step.
• ...and so on...

48
Climbing Stairs Again

• while (Im not at the top of the stairs)
• Climb up one step
• Climbing stairs is a while loop!

49
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter gt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• change termination condition

50
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter gt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• change termination condition
• body of loop never executed

51
Using while Statements

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter gt counter)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• change termination condition
• always true

52
Infinite Loops

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter gt counter)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 1
• System.out.println("End of demonstration")
• if termination condition always true, loop never
  ends
• infinite loop goes forever

53
Infinite Loops

• public class WhileDemo
• public static void main (String args)
• int limit 3
• int counter 1
• while (counter lt limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter - 1
• System.out.println("End of demonstration")
• good termination condition
• but process never gets closer to condition

54
Infinite Loops

• public class WhileDemo
• public static void main (String args)
• int limit 9
• int counter 0
• while (counter ! limit)
• System.out.println("The square of "
  counter
• " is " (counter
  counter))
• counter counter 2
• System.out.println("End of demonstration")
• process gets closer to termination condition
• but never satisfies condition, keeps going past
  it

55
Another while Example

• public class PrintFactorials
• public static void main (String args)
• int limit 10
• int counter 1
• int product 1
• while (counter lt limit)
• System.out.println("The factorial of "
  counter
• " is " product\)
• counter counter 1
• product product counter
• System.out.println("End of demonstration")
• accumulate product

56
Questions?